Several new features are added for the long term HSPA (high speed packet access) evolution in order to meet the requirements set by the IMT-A (International Mobile Telecommunications Advanced). The main objective of these new features is to increase the average spectral efficiency.
One possible technique for improving downlink spectral efficiency would be to introduce support for 4-branch MIMO (multiple input multiple output), i.e., utilize up to four Tx (transmit) and Rx (receive) antennas to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. 4-branch MIMO provides up to 84 Mbps per 5 MHz carrier for high SNR (signal-to-noise ratio) users and improves coverage for low SNR users.
The current HSDPA system (Release 7-10) supports one or two transmit antennas at the Node B. For these systems, from channel sounding, the UE (user equipment) measures the channel and reports the channel state information in one sub frame to the Node B as feedback. Typically this report includes a CQI (channel quality indicator) which explicitly indicates a preferred rank and a PCI (precoding control indicator). The UE sends this report periodically for every subframe or TTI (transmit time interval) as feedback. Once the Node B receives this report, it grants modulation and coding, number of codes, rank and the precoding channel indicator to each specific UE based on a scheduler metric.